Conventionally, various types of sensors have been proposed that detect a collision of a vehicle with an object. For example, US 2005/0116817 corresponding to JP-A-2005-538881 discloses a front sensor having a chamber room where a sensing element is placed. The front sensor determines that a collision occurs, when the sensing element detects a pressure change in the chamber room due to deformation of the chamber room. US 2006/0087417 corresponding to JP-A-2006-117157 discloses an object determination sensor that determines what kind of object a vehicle collides with based on a pressure change in a sealed chamber room located between a bumper cover and a bumper reinforcement. However, pressure in such a chamber room may change due to factors other than a collision. For example, a vehicle vibration and a temperature change may cause a pressure change in the chamber room.